Health insurance question, does anyone have experience with Health Savings Accounts (HSA) versus Health Reimbursement Accounts(HRA)?

It’s open enrollment at my s/o’s company and these are the two choices. The employer is pushing employees towards the HSA plan, claiming the estimated annual cost to the employees is substantially less than the HRA. It appears to me that if you lived until retirement without a single medical issue that’s probably true. But it looks like someone could get hammered financially if they had an issue and substantial medical expenses over time with the HSA. Does anyone have real life experience with HSAs?